Occam's Razor

Occam's Razor (or Ockham's Razor) is the philosophical principle which states:
entities should not be multiplied beyond necessity.

Explanation

In layman's terms this means "Out of several equally good explanations, pick the simplest one." In this definition, the word "simplest" means "the explanation that contains the fewest assumptions." Similarly, "equally good" refers to the ability of the explanation to account for the observation and not to the veracity of the explanation.

Example

Assume a child contracts a nasty disease which leaves her in terrible pain. She is taken to the doctor and is given medicine. Meanwhile, the parents pray to God for her safe recovery. A few days later she is perfectly healthy again and the parents, in their happiness, proclaim "God saved our child!"

A better explanation for the child's recovery, however, would be that the medicine the doctors gave her did its job. When one compares the two explanations ("God did it" and "The medicine worked") it is clear that the second requires fewer assumptions. God isn't required for the medicine to have worked in the second explanation, therefore it is the one that should be chosen.

Extending the Razor

Consider any observation which requires a causal explanation; a rainbow, for example. We can devise numerous explanations for this phenomenon:

Yahweh creates them as a reminder of his promise to never again flood the entire earth

Light dispersion due to refraction as it passes through water droplets

Each of these explanations answers the question and they all also prompt additional questions, but only the second answer explains the phenomena in terms which we adequately understand. The additional questions raised by the second explanation (dispersion, refraction, visible spectrum, etc.) are simpler concepts supported by consistent, reliable definitions.

Each of the other explanations could be considered equivalent non-answers. The causal subjects are complex constructs which prompt many additional, unanswered questions about the nature of those subjects and how they managed to create the rainbow. The fact that these explanations are non-answers can be made more clear by substituting the subjects into different answer:

Yahweh creates them to mark pots of gold

Leprechauns sprinkle colored dust in the sky

Sky pixies create them as a reminder of their promise to never again flood the entire earth